U.S. Pat. No. 4,832,442, dated May 23, 1989, issued to Vincent J. Pappas, incorporated herein by reference, discloses a long distance transmission fiber optic cable system having a strength messenger with a fiber optic cable helically wrapped around said messenger. Messenger and communication cable are then placed on a reel. The reel is placed on a truck and then the truck moves to the installation site. Pappas then states that the cable system is simply unwound and suspended in catenary curves between spaced, elevated outdoor supports.
In practice, such installations, as well as other aerial cable installations, are accomplished by mounting sheaves on the poles or towers. A pulling rope, such as a nylon rope, is fastened to the strength messenger cable and then placed over the sheaves on the towers in sequence until the desired pulling point is reached. The pulling rope is then pulled at the pulling point, unwinding the cable system from the reel and drawing the cable system from sheave to sheave until the destination is reached.
It is further customary for the pulling rope to be attached to the cable being pulled by means of a swiveled pulling grip. It is generally undesirable to impart rotational torque on any cable, and the swivel at the point of attachment between the pulling rope and the cable allows such torque to be dissipated at that point. Such swivels are generally adequate to solve the problems of torque when installing a single cable by this method.
Applicants have found that the use of such a swivel at the pulling end is not sufficient to address the problem of torque in the installation of a dual cable system such as described by Pappas. Although torque can be dissipated at the pulling end where the swivel is located, this has been found insufficient to address the problem of torque between the towers or poles. This is especially worrisome in the installation of helically wrapped dual cable systems because both cables tend to rest against the sheave while under tension in the pulling process. As the dual cables are being pulled, a worm gear like action results, imparting torque to the cable system both in front of and behind the sheave. Although such torque at the front end is dissipated at the swivel, the swivel can do little or nothing to relieve the torque building up between the sheave and the preceding sheave or the unwinding reel. The building up of unrelieved torque in turn can cause difficulties in pulling the system and could even damage the transmission cable. Such a transmission cable is not usually designed to be especially rugged since it depends on the messenger for strength purposes.
Applicants believe that the use of a smaller sheave will not be sufficient to fully address the problem, because both cables might rest against the sheave perimeters which define the rim. The use of very narrow sheaves would also tend to lead to increases in frictional drag during the pull